Weekend downpour nudges Sydney closer to setting various rainfall records as La Niña appears on the horizon.
As New South Wales experiences another deluge, updated predictions are heightening the likelihood of La Niña making a comeback in 2024—the weather phenomenon that contributed to Sydney’s record-breaking rainfall just two years ago.
While La Niña would significantly raise the chances of above-average precipitation across Australia in the latter part of the year, a separate weather system is already saturating the state this weekend, prompting flood warnings and leading to another overflow of Warragamba Dam.
This current rainfall event is notably extensive, driven by moist easterly winds colliding with a passing low-pressure trough from the west—mirroring the same weather pattern that brought rain last week.
Extensive rainfall across the state leads to a waterlogged Sydney, potentially setting a record for the wettest April-May period.
By Friday evening, Observatory Hill had accumulated nearly 500 millimeters of rain over the past six weeks, marking the wettest April-May period in 21 years. With three weeks still remaining in the month, Sydney’s April-May rainfall record of 673mm from 1988 is certainly in jeopardy, especially as rain continues to fall over the weekend. Another potential record on the horizon is Sydney’s longest streak of consecutive rainy days, defined by Observatory Hill receiving at least 1mm of rain. The city’s current streak stands at 11 days as of Friday, just five days shy of the record set in 2022 and 1943. The rainfall expected over the weekend should extend the streak to 13 days, and the possibility of setting a new record exists if isolated showers throughout the week happen to pass over the Observatory Hill rain gauge.
As for the weekend rainfall forecast, Sydney could easily accumulate another 100mm by Monday, with up to 300mm possible along the state’s southern coast, prompting flash flood warnings in parts of the Illawarra and South Coast. The Bureau of Meteorology (BOM) has also issued a flood watch for several rivers due to ongoing rain falling on already saturated catchments, including the Hawkesbury-Nepean where flows will be increased by a second spill of Warragamba Dam this year, although NSW Water expects the severity to be significantly lower than in early April.
NSW Water stated, “The dam could reach a peak spill rate of about 40 gigalitres per day (GL/day), well below the spill peak experienced in early April, which exceeded 200 GL/day.” While the persistent wet weather may be causing inconvenience in Sydney, the autumn rain is beneficial for agriculture, with modeling indicating that many western areas can expect over 50mm—equivalent to around two months’ worth of rain—thus constituting an ‘autumn break’, which replenishes soil moisture for winter cropping.
Comparing Sydney’s rainfall in 2024 to the record wet year of 2022.
Sydney has experienced rainfall on nearly half of the days in 2024, resulting in a total accumulation already surpassing 800mm. This amount accounts for roughly two-thirds of the annual average and significantly exceeds the typical amount of about 600mm by the end of May. While this year has been notably wetter than usual, the intensity of rainfall has been considerably lower compared to 2022, when a staggering 1,531mm had accumulated in the first five months alone. In 2022, the total rainfall at Observatory Hill reached 2,530mm, marking the highest annual total since records began in 1859.
The amount of rainfall for the remainder of 2024 will largely hinge on ocean temperatures across the Pacific.
The probability of La Niña is close to 90 percent, according to the US weather agency.
Global weather patterns over the next 12 months are delicately poised, but the National Oceanic and Atmospheric Administration (NOAA) strongly indicates a likelihood of a shift towards La Niña. According to NOAA, “The most recent International Research Institute plume favors an imminent transition to ENSO-neutral, with La Niña potentially developing during July-September 2024 and continuing through the Northern Hemisphere winter,” as outlined in their monthly Pacific diagnostic.
NOAA asserts that La Niña is the most probable outcome for this year, with a return of El Niño considered highly unlikely. La Niña involves a cooling of the central equatorial Pacific, which impacts global weather patterns, including increased cloud formation and rainfall around Australia. NOAA’s confidence in La Niña stems primarily from computer modeling, but historical trends also support this outlook: “La Niña typically follows strong El Niño events, which further reinforces the reliability of the model predictions,” notes NOAA.
As evidence of La Niña’s potential, NOAA’s official probability of its occurrence exceeds 80 percent by spring and rises to nearly 90 percent by December. Furthermore, the Pacific Ocean is already displaying signs of transitioning into a cooler phase—the sea surface temperature in the crucial region influencing El Niño and La Niña episodes has decreased by more than 1 degree Celsius between November and April. These indicators lend substantial support to the predictions of La Niña’s development.
Ocean temperatures in parts of the NINO3.4 box have already fallen below average.
The final uncertainty lies in whether the current cooling trend will progress into a complete La Niña event as forecasted by models, or if it merely represents a regression from the recent warm El Niño episode.
Record high ocean temperatures pose a significant challenge.
Assessing this year’s rainfall potential involves considering the impact of climate change, which not only directly influences weather patterns but also shifts the goalposts for defining La Niña. Before the prominence of global warming, categorizing Pacific phases was straightforward: El Niño occurred when the NINO3.4 region was warm, and La Niña occurred when it was cold, with the Bureau of Meteorology (BoM) using thresholds of 0.8°C above (El Niño) or below (La Niña) the average.
The latest weekly reading of water temperature in the key monitoring region, the NINO3.4 value, stands at 0.73°C above the 1961-1990 baseline average, falling just within the BoM’s neutral range, indicating that we are currently experiencing neither La Niña nor El Niño. However, global ocean temperatures are currently at their warmest on record, around 1°C above this baseline average. Therefore, compared to the rest of the world, the NINO3.4 region appears colder than normal.
A graph depicting the traditional NINO3.4 value alongside a modified figure called a relative index, which eliminates the influence of climate change, shows that the Pacific region is already much closer to La Niña than initially analyzed. This underscores the importance of considering the evolving dynamics of climate change when interpreting traditional weather patterns such as La Niña.
Indeed, weather agencies such as NOAA and the BOM may need to adopt a flexible approach in their analysis of La Niña’s onset this year, one that can adapt to the rapidly warming conditions of our world. With climate change influencing weather patterns and altering traditional indicators, it’s crucial for meteorological organizations to remain dynamic in their assessments and interpretations, ensuring their analyses reflect the evolving realities of our changing climate.
Leave a Reply